1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic technology, and in particular to an image forming apparatus provided with a charge removing means for radiating light of a specific wavelength range to a photosensitive film located on a surface of a photosensitive member.
2. Description of the Related Art
Conventionally, image forming apparatuses using electrophotographic technologies have been actively developed for use as, for example, electrostatic copiers or printers.
Briefly referring to FIG. 6, a conventional image forming apparatus using an electrophotographic technology will be described. An image forming apparatus 1 includes a rotatable photosensitive drum 3 having a photosensitive film 2 located on a surface thereof, a main charger 4 for uniformly supplying the photosensitive film 2 with a prescribed level of electric charge, an optical device 5 for exposing the photosensitive film 2 and forming an electrostatic latent image on the photosensitive film 2, a developing device 6 for developing the electrostatic latent image formed on the photosensitive film 2 into a toner image, a transfer device 8 for transferring the toner image on the photosensitive film 2 onto a recording paper sheet 7, a cleaning device 9 provided with a cleaning blade for removing the residual toner on the photosensitive film 2, and a charge removing lamp 10 for removing the residual charge on the photosensitive film 2 and thus setting the surface potential of the photosensitive film 2 at a prescribed uniform level.
In the image forming apparatus 1 having the above-described structure, an image is formed in the following manner.
First, the main charger 4 supplies the photosensitive film 2 on the photosensitive drum 3 with a prescribed uniform charge. Next, light is radiated to the photosensitive film 2 by the optical device 5 to form an electrostatic latent image on the photosensitive film 2. Toner is supplied to the photosensitive film 2 by the developing device 6 to develop the electrostatic latent image into a toner image. The toner image on the photosensitive film 2 is transferred to the recording paper sheet 7 by the transfer device 8. After the transference, the residual toner on the photosensitive film 2 is removed by the cleaning device 9. Light is radiated on the photosensitive film 2 by the charge removing lamp 10 to remove the residual charge on the photosensitive film 2. Thus, the surface potential of the photosensitive film 2 is set at a prescribed uniform level. Thereafter, the photosensitive film 2 is charged again by the main charger 4. Such a process is repeated in accordance with the rotation of the photosensitive drum 3.
The photosensitive film 2 is formed of an inorganic or an organic material. Usable inorganic materials include, for example, Se-type materials and amorphous Si-type materials.
Recently, more and more photosensitive films are formed of an organic material due to high safety and easy processibility thereof. Photosensitive bodies formed of an organic photosensitive material are classified into multiple-layer organic photosensitive bodies and single-layer organic photosensitive bodies.
A multiple-layer photosensitive body includes a charge generating layer and a charge carrying layer which are laminated on a substrate. The charge carrying layer contains a charge carrying material. The charge carrying material may be a hole carrying material or an electron carrying material. There are various hole carrying materials which have a satisfactory carrying ability, whereas no electron carrying material having a satisfactory carrying ability has been developed. Accordingly, multiple-layer organic photosensitive bodies are mostly negatively charged. However, when a photosensitive body to be negatively charged is charged using a charger for performing discharge by way of corona discharge, ozone is generated. In order to prevent exposure of the human body to ozone and to protect the environment, an additional measure to deal with ozone is needed.
In an attempt to solve the above-described problem, single-layer organic photosensitive bodies have been developed. A single-layer organic photosensitive body contains a charge carrying medium which is formed of a binder resin containing a charge carrying material dispersed therein and further a charge generating material dispersed in the charge carrying medium. A photosensitive body to be positively charged can be easily formed of a single-layer photosensitive body containing a charge carrying material having an electron carrying ability.
FIG. 7 is an enlarged cross sectional view of an important part of the photosensitive drum 3 which is formed of a single-layer organic photosensitive body. The photosensitive drum 3 includes a substrate 3a formed of, for example, an aluminum tube and the photosensitive film 2 formed of a single-layer organic photosensitive body which is laminated on the substrate 3a. The photosensitive film 2 is formed of a charge carrying medium 2a, which is formed of a binder resin containing a charge carrying material dispersed therein and further a charge generation material 2b dispersed in the charge carrying medium. When light is radiated to and incident on the photosensitive film 2, the charge generating material 2b generates carrier pairs each having a hole and an electron.
When the photosensitive film 2 having such a structure is positively charged, positive charges are distributed on a surface of the photosensitive film 2. When light in accordance with an image is radiated to the photosensitive film 2 which is positively charged, the carrier pairs are generated. Among the carrier pairs, electrons generated in the photosensitive film 2 reach the surface of the photosensitive film 2 and are bound with the holes. In this manner, an electrostatic latent image is formed.
A single-layer organic photosensitive body is easier to produce than, and thus is preferable to, a multiple-layer organic photosensitive body. However, the photosensitive film 2 formed of a single-layer organic photosensitive body to be positively charged has a problem in that electrons which are generated as photocarriers by the charge generation material 2b tend to remain therein because the charge carrying medium 2a is low in the electron carrying ability. Due to such inferiority in the electron carrying ability of the charge carrying medium 2a, the photosensitive film 2 still retains a generally high charge even after being exposed to light by the charge removing lamp 10. If the photosensitive film 2 is positively charged in the state of having electrons therein, the electrons move to the surface of the photosensitive film 2 and are bound with the holes which are charged on the surface of the photosensitive film 2. As a result, the surface potential of the photosensitive film 2 is reduced, and thus the following inconveniences occur.
FIG. 8 is a graph illustrating the potential which is obtained when the process of charging-developing-removal of the charge is repeated with no exposure. The potential at the time of developing is indicated by symbol SP, and the potential immediately after the removal of the charge is indicated by symbol RP. As is appreciated from FIG. 8, when removal of the charge is insufficient, the charging potential is also insufficient. The potential SP at the time of developing reduces cycle by cycle, and the potential RP after the removal of the charge rises cycle by cycle. Accordingly, in the case that an image is to be formed by multiple rotations of the photosensitive drum 3 having the photosensitive film 2 with a relatively large amount of residual electrons as photocarriers, the density of the image changes rotation by rotation of the photosensitive drum 3, thus causing non-uniformity in the image density. Furthermore, such a large amount of electrons residual in the photosensitive film 2 reduces the surface potential at the time of developing. Especially when the main charger 4 is Scorotron, the photosensitive film 2 is charged so as to have a uniform surface potential. Accordingly, if the surface potential is reduced by the residual electrons at the time of charging, the main charger 4 performs discharge so as to compensate for the reduction in the surface potential. This increases the power consumption by the main charger 4.